1. Field of the Invention
The present invention relates to a method for forming a silicon-based deposited film and a method for forming a photovoltaic element, such as a solar cell, including a silicon-based deposited film
2. Description of the Related Art
Radio-frequency plasma chemical vapor deposition (RF plasma-enhanced CVD) serves as an excellent mass-production method for silicon-based deposited films from the viewpoint of improvement in throughput and the ease of low-temperature formation and an increase in area. With respect to solar cells as examples of products including silicon-based deposited films, solar cells including silicon-based deposited films have the following advantages over existing energy generation devices utilizing fossil fuels: an inexhaustible energy source and clean power generation processes. To expand the use of solar cells, however, it is necessary to further reduce costs. To achieve the cost reduction, an increase in deposition rate by RF plasma-enhanced CVD and the establishment of the technique for further improving characteristics are important technical issues.
For example, Japanese Patent Laid-Open No. 11-330520 discloses a method for producing a microcrystalline silicon deposited film. Japanese Patent Laid-Open No. 11-330520 discloses a technique for producing a silicon-based deposited film under conditions in which an atmosphere contains a silane gas and a hydrogen gas, the pressure in a reaction chamber is 5 Torr or more, and the distance between a substrate and an electrode is within 1 cm. The patent document discloses that using the method, the film can be formed at a high deposition rate, and a photoelectric conversion apparatus including the film has high conversion efficiency.
Japanese Patent Laid-Open No. 2000-252484 discloses deposited film-forming conditions in which the partial pressure of a SiH4 gas is set in the range of 1.2 Torr to 20 Torr, the distance between electrodes is set in the range of 8 mm to 15 mm, and the amount of hydrogen diluent gas is equal to or less than four times the amount of the SiH4 gas.
Japanese Patent Laid-Open No. 11-243219 discloses a layered photovoltaic element including a component having a pin junction with an i-type semiconductor layer composed of a microcrystalline semiconductor material, wherein a current is controlled by the component, the degradation of the layered photovoltaic element by light is suppressed, and the performance of the photovoltaic element is improved.
Japanese Patent Laid-Open Nos. 3-70183 and 2002-299658 and Japanese Patent Publication No. 3271990 each disclose a technique for stacking a microcrystalline silicon thin film on a single-crystal silicon substrate or a polycrystalline silicon substrate by plasma-enhanced CVD to improve performance.
The characteristics of silicon-based deposited films formed by plasma-enhanced CVD have been gradually improved by the techniques disclosed in the above-described patent documents. For example, in plasma-enhanced CVD, a relatively high pressure (e.g., 600 Pa or more) and a small distance between electrodes (e.g., 10 mm or less) compared with those in the related art are used. Under such conditions, a deposited film containing microcrystalline silicon and having relatively excellent characteristics can be formed at a high deposition rate of 1 nm/s or more. Elements, for example, photovoltaic elements such as solar cells, produced by these techniques have improved conversion efficiency, suppressed degradation rates, and higher performance.
To establish further improved performance and cost reduction, there are still problems to be solved.
For example, a roll-to-roll plasma-enhanced CVD apparatus and a plasma-enhanced CVD apparatus in which a crystalline substrate is held by a holder and a surface of the substrate is subjected to plasma treatment while the holder is conveyed will be described. As one of the features of such an apparatus for continuously forming a deposited film on the substrate being conveyed, the deposited film can be continuously formed for a prolonged period (e.g., 30 hours or more). Thus, the apparatus has advantages of high throughput, high productivity, ease in increasing area, and the like.
The characteristics and uniformity of the deposited film may be varied with time to form the deposited film. For example, in the case of the continuous formation of the deposited film for a prolonged period, deposited films are also formed on surfaces of inner walls of a deposited-film-forming chamber and surfaces of radio frequency electrodes. Thus, the distance between the surface of the electrode and the substrate is reduced by the thickness of the film formed on the surface of the electrode, thereby reducing the volume of the space (film-forming space) in the deposited-film-forming chamber. Furthermore, the deposited films affect the resistivity of the surfaces of the electrodes and the inner walls, thus changing the plasma state.
For example, in the case where the distance between the electrodes is 10 mm, when the deposited-film-forming time is 30 hours or more, the thickness of the film deposited on the radio frequency electrode is increased to about 0.5 to 1.0 mm, depending on conditions. Since the space is reduced by the thicknesses of the deposited films on the surfaces of the radio frequency electrodes, the volumetric capacity of the deposited-film-forming chamber is substantially reduced by 5% to 10% of the initial volumetric capacity. Furthermore, the deposited films on the surfaces of the inner walls of the deposited-film-forming chamber and the surfaces of the radio frequency electrodes each function as a resistor of some kind. Thus, the size of a plasma sheath, a bias current, a bias voltage (including a self-bias voltage), or the like is changed, thereby possibly changing the distribution of a plasma potential, a plasma state, or the like. Specifically, for example, in the case where a deposited film containing microcrystalline silicon is formed on a substrate, an increase in the thickness of films deposited on the radio frequency electrodes changes the potential distribution in the deposited-film-forming chamber (plasma space). The resulting potential difference may increase the probability of ion bombardment in which the substrate is bombarded with cations present in the plasma space. The bombardment of the cations disturbs the structure of the deposited film, thus possibly reducing the crystallinity of a specific region of microcrystalline silicon or causing the amorphization of microcrystalline silicon. Therefore, a region composed of amorphous silicon is partially or entirely formed on the substrate.
Also in the case where a deposited film is intermittently formed with an interruption of the deposition for a predetermined period of time in the course of a process, the plasma state is changed. For example, such a process includes interrupting the deposition after a deposited film is formed on a first substrate and then resuming the process after the substrate is replaced with a second substrate. In such a case, the change of the plasma state with respect to the cumulative deposition time is larger than that in the case where the deposition is uninterruptedly conducted. This may be due to the fact that a discontinuous interface is formed in the films deposited on the substrate and the radio frequency electrodes in the thickness direction.
In the method for forming a deposited film by roll-to-roll plasma-enhanced CVD, a sheet substrate may be used. Such a substrate is relatively thin (for example, 1 mm or less). General examples of the material of the sheet substrate include metals such as stainless steel; and resins such as aramid resins and polyimide resins. When the sheet substrate is used, in some cases the sheet substrate is deformed by, for example, tension applied to the substrate during the motion (or conveyance) of the substrate, thermal expansion due to heat from a heater and a plasma, or the internal stress in the deposited film on the substrate. For example, the substrate is arranged such that a deposited-film-forming side of the substrate faces downward (in the direction of gravitational force) and that the substrate faces the radio frequency electrodes. In the case of the conveyance of the substrate, the middle portion or ends of the substrate in the width direction are partially curved in some cases.
In cases of a sufficiently large interelectrode distance (e.g., 50 mm or more) or reduced thicknesses of films deposited on the surfaces of the radio frequency electrodes and the inner walls of the deposited-film-forming chamber, such deformation of the substrate has only a small effect on the plasma state.
In the case of the deposited-film-forming chamber having an interelectrode distance of about 10 mm or less, however, even a slight change in interelectrode distance due to the deformation of the substrate relatively increases the rate of change, thus possibly degrading the uniformity of the plasma (increase in nonuniformity). Furthermore, the thickness (e.g., 1 mm or more) of each of the films deposited on the surfaces of the radio frequency electrodes and the inner walls of the deposited-film-forming chamber is increased with deposition time to undesirably affect the uniformity of the plasma. When a change in interelectrode distance is 1 mm, the rate of change in interelectrode distance is as high as 10% with respect to the entire interelectrode distance (10 mm). That is, the interelectrode distance is changed by 10% of the initial distance. This locally changes the state of plasma on the surface of the substrate, thereby increasing the nonuniformity of the plasma as a whole.
The nonuniformity of the plasma state may locally change the crystal system of the film deposited. For example, although a deposited film is formed under conditions suitable for the formation of microcrystalline silicon, a deposited film composed of amorphous silicon can be locally formed. The area ratio of a region where the deposited film composed of amorphous silicon is formed to a region where the deposited film composed of microcrystalline silicon is formed increases with time. In the present invention, the regions described above are also referred to as a “region composed of microcrystalline silicon” and a “region composed of amorphous silicon”.
In other words, the ratio of the area of the region composed of microcrystalline silicon to the total area (the sum of the areas of the region composed of microcrystalline silicon and the region composed of amorphous silicon) is reduced with time.
In the case where the deposited film is formed continuously and stably under conditions maintained within a predetermined range, rapid changes do not easily occur. Thus, the changes are not easily elicited because the deposited film is less affected by the changes (a small change in characteristics) in the short term. However, the change in the area ratio of the region composed of amorphous silicon to the region composed of microcrystalline silicon gradually changes the characteristics. The change in characteristics increases with time and becomes apparent. In the case of the continuous formation of the deposited film for a prolonged period of time, the proportion of the region composed of microcrystalline silicon varies depending on the stage of the film-forming process; that is, whether one is in an early, middle, or late stage of the film-forming process. Thus, the characteristics of the deposited film also vary.
As described above, in the case where the deposition is interrupted in the course of the process and is then resumed after a predetermined period of time, the change of the plasma state with respect to the cumulative deposition time is larger than that in the case where the deposition is conducted without interruption. Thus, the yield and the like are markedly affected. This may be because an interface is formed in each of deposited films on the surfaces of the radio frequency electrodes and the inner walls of the deposited-film-forming chamber by interrupting the deposition. This is believed to be due to the fact that the interface state of the deposited film is increased which changes the resistivity, adhesion, surface shape of the deposited film and provides a discontinuous interface.
In the deposited film containing microcrystalline silicon, an increase in the area ratio of the region composed of amorphous silicon to the region composed of microcrystalline silicon changes the state (e.g., grain size, crystallinity, hydrogen content, or defect density) of the microcrystalline silicon in the deposited film, thus degrading the characteristics and uniformity. When such a deposited film containing microcrystalline silicon is used as the photoactive layer of a photovoltaic element, the design characteristics (e.g., open-circuit voltage, short-circuit current, fill factor, conversion efficiency, and degradation rate by light) of the element are not obtained in some cases.
In recent years, photovoltaic elements such as solar cells have been required to have higher performance, and the allowable variation range of characteristics has become smaller. Thus, the change in characteristics significantly reduces the yield.
As employed herein, the phrase “crystalline substance” refers to a microcrystalline substance and the phrase “different crystal systems” includes microcrystalline systems and amorphous systems.